Why does a 18v supply suddenly become a 25v supply with a power supply cap?

[Absentidei]

I'm working on a spring reverb.
To drive the spring I have a lm386 chip running from a 18v power supply.
I use a tl072 powered from a 5v regulator as a buffer and to split the dry and wet signal.

It seems the power supply I have is really REALLY crappy, cause it makes the the op amp go "skreeeeeeeeeeeech".
As in: REALLY REALLY NOISY.

When I stick a 47uf capacitor across the hot and ground on the power supply, the op amp starts to behave as expected, but when I measure the voltage across the hot and ground rail on my breadboard I measure 25v...
Where do those extra 7v come from?


Also, I'm using this jfet booster to boost the signal coming from the reverb tank up to line level before it hits the blend pot...
http://www.beavisaudio.com/techpages/blocksfragments/images/JFET_Booster_Schematic.gif
But since it's now running from 18v and that schematic is made for 9v, it just makes a tone.
What resistors do I need t change to get it to work from 18v?


Edit:
The tone I get from the op amp without the filtering cap across my power supply is 100 Hz, wich makes sense since the AC power in norway is 50Hz.
But why does that ripple go trough the L7805cv voltage regulator? I thought voltage regulators were supposed to deliver clean DC?

[R.G.]

....It seems the power supply I have is really REALLY crappy....

Good guess.

When I stick a 47uf capacitor across the hot and ground on the power supply, the op amp starts to behave as expected, but when I measure the voltage across the hot and ground rail on my breadboard I measure 25v...
Where do those extra 7v come from?

I think it's Highly Likely that the power supply you're using has rectification, but no filtering. This is especially likely since 18V * 1.414 = 25.4, which is suspiciously like the voltage you get.

The DC value of a full wave rectified, but unfiltered "18V" supply is far below the peak value. Adding a capacitor pulls this average up to near the peak, about 25V.

There are other possibilities, but I'd start with this one.

[Absentidei]

Thanks a bundle!

I found a different 14v power supply that seems to have proper filtering, and with that everything seems to work a lot better!

Now I just need to figure out a way to boost the signal after the reverb tank back up to line level.